Blaine HansonDepartment of Land, Air and Water Resources

University of California, Davisbrhanson@ucdavis.edu

Irrigation of Agricultural Crops in California

What percentage of California’s water is used by agriculture?

80 %: based on the developed water supply52 %: based on the total water supply of a dry year29 %: based on the total water supply of a wet year

Davis 2001 (wet year)

Day of year

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375

Pote

ntia

l eva

potr

ansp

iratio

n (in

ches

per

day

)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Rainfall (inches)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6EvapotranspirationRainfall

Why irrigate?

Water Use of California Crops (3 year Average)

0 1,000 2,000 3,000 4,000 5,000 6,000

Grain

Rice

Cotton

sugarbeet

Corn

Dry Bean

Safflower

Other Field

Alfalfa

Pasture

Tomato (Pr)

Tomato (Fr)

Curcurbits

Onion/Garlic

Potato

Other Truck

Almond/Pistachio

Other Decid. Tree

Subtropical Tree

Vine

Water Use (AF/year X 1,000)

Alfalfa

Forages

Water Use of California Crops

How much water does agriculture need?

What is evapotranspiration (ET)?

Evapotranspiration: crop water useWater evaporation from plant leaves (transpiration)Water evaporation from soil surfaceMore than 95% of the water uptake by plants is evaporated

FactorsClimate: solar radiation, temperature, humidity, windPlant: crop type, stage of growth, healthSoil moisture content

Units of evapotranspiration (ET)

Volume of waterOne acre-inch = 27,160 gallonsOne acre-foot = 325,900 gallons

Depth of water (inches, feet, cm, mm)Standardized water use (independent of field size)One inch of water = 1 acre-inch per acre = 27,160 gallons per acre

Measuring evapotranspiration (ET)Difficult and expensive to measureMethods

Lysimeter – very expensive, restricted to ag field stationsMeteorological methods – moderately expensive, portableSoil moisture measurements – inexpensive, can be inaccurate

California Irrigation Management Information System (CIMIS)

Network of weather stations Installed and maintained by the University of California and California Department of Water ResourcesWeather data used to calculate a reference crop ET (ET of grass or alfalfa) Crop coefficients (Kc) used to relate reference crop ET to actual crop ETET = Kc x Reference crop ET

Lysimeter

Meteorological Methods

Soil moisture measurements

CIMIS weather station – data and complex equations are used to calculate a reference crop ET

Crop ET = crop coefficient x reference crop ET

Evapotranspiration of selected crops

Alfalfa

Where do dairy products come from?

Dairy products: ice cream, cheese, milk, yogurt, butterDairy cows produce the milk used to make these productsDairy cows eat about 70% of the alfalfa produced in California

Alfalfa

Products: ice cream, milk, cheese, yogurt, butterSeasonal ET of alfalfa = 55 inches of water = 55 acre-inches per acre = 1,500,000 gallons per acre160 acres: ET = 160 acres x 1,500,000 gallons per acre = 240,000,000 gallons of water per year (does not included irrigation system inefficiencies)Are we wasting water growing alfalfa?

Grain

Products: bread products, rice, cereal, chicken, eggs, steakSeasonal ET of wheat = 16 inches of water = 16 acre-inches per acre = 435,000 gallons per acre160 acres: ET = 160 acres x 435,000 gallons per acre = 69,600,000 gallons of water per year (does not included irrigation system inefficiencies)

What crops should be grown in California?

Maximize dollar returns?

Only high cash value crops should be grownTree cropsVegetable cropsTomatoes

Low cash value crops should not be grownWheatCornCottonAlfalfa?

Maximize human health?

The situation

Agriculture cannot compete economically with the urban/industrial sector for water.

Uses a large amount of water per unit of productionWe do not pay very much for the agricultural products

Regardless of the economics, if we want food we will have to pay the price in terms of water and land for producing the agricultural products used to produce our food. There is no other choice if we want food!Lower-cash value crops provide a major part of our diet

Irrigation methods in California

Irrigation efficiency

Definition: ratio of water beneficially used to amount appliedBeneficial uses

ET – major useSalinity controlFrost protectionDrip system maintenance

Losses affecting the irrigation efficiencySurface runoff – water that runs off the lower end of gravity irrigated fieldsDeep percolation – water that percolates through the soil below the root zoneEvaporation

Different numbers for farm, irrigation district, regional irrigation efficiencies

Furrow irrigation (gravity)

Flood or border irrigation (gravity)

Wheel-line sprinkle system

Hand-move sprinkle system

Portable solid-set sprinkler system

Center-pivot sprinkler system – inappropriate for many California soils

Linear-move sprinkler system

Microsprinklers – tree crops

Microsprinkler

Drip irrigation – vineyards,tree crops

Drip emitter

Drip irrigation – row crops

Surface drip irrigation

Subsurface drip irrigation

Drip tape

Which irrigation method is the best?Gravity irrigation

Low capital costLow labor cost to operateDifficult to manage efficiently – trial and error approachSurface runoff can cause water quality problems

Sprinkler irrigationModerate capital costLow to moderate labor costs to operateEasy to manageEfficiency limited by wind effects

MicroirrigationHigh capital costs (up to $1,000 per acre)Precise application of water throughout a fieldModerate labor costsEasy to manage Highly susceptible to emitter clogging

Maximum potential irrigation efficiencies

Irrigation method Irrigation efficiency (%)Gravity (furrow, flood) 70-85

Sprinkle

Hand-move, wheel-line, solid set 70-80 (low wind)

Center pivot, linear-move 80-90

Microirrigation 80-90

Will increasing the farm irrigation efficiency save water that can be used elsewhere?

Numerous studies have attempted to answer this questionMany researchers are not very familiar with irrigated agricultureSome ignore reality Questionable assumptions, results, and conclusions

Problem – losses from one farm frequently are used by downstream water users

Difficult to track where the water goes Little or no real water savings

Two farm irrigation district

ET = 25 ac-ftIE = 50 % IE = 100%

100 ac-ft

25 ac-ft

100 ac-ft

ET = 25 ac-ft

ET = 25 ac-ft ET = 25 ac-ft

IE = 50 % IE = 100%

50 ac-ft

50 ac-ft

75 ac-ft

50 ac-ft

50 ac-ft

25 ac-ft

25 ac-ft

25 ac-ft

25 ac-ft

50 ac-ft

75 ac-ftSurface runoff = 0 ac-ft

Surface runoff =25 ac-ft

Surface runoff =25 ac-ft

Surface runoff = 0 ac-ft

Estimates of potential water savings from  increased irrigation efficiency

University of California study – 843,000 acre-feetUniversity , state and federal agencies, irrigation districts, grower organizationsConsidered reuse of waterEstimate based on amount of water not reused downstream

Consultant study – at least 4.400,000 acre-feetDid not consider reuse of water

Improved water quality may be the primary benefit of increased irrigation efficiency rather than water savings – reduced surface runoff (sediments, pesticides, nutrients)

Where will the water come from?

No more dams for water storageWater conservation from increased irrigation efficiency?Removal of agricultural land from production – most likely source of water for satisfying the increased urban/industrial and environmental water demands

DWR water transfer programMWD program of removing alfalfa fields from production on a rotating basis in the Palo Verde Valley – water is transferred to the LA area

Deficit irrigation of agricultural fieldsRegulated deficit irrigation – trees and vine crops (UC Davis)Mid-summer deficit irrigation - alfalfa (UC Davis)

Reduced urban/industrial and environmental demands

Summary

Agriculture is California’s largest user of water.It takes a lot of water to produce a crop.The price that society has to pay for food is the water and land required to produce the crops needed for food. There is no other choice. It is unlikely that increasing irrigation efficiency will have a large impact in supplying the predicted future water needs of the urban/industrial and environmental sectors. Agricultural land will need to be removed from production to supply the needed water.

Have a good day!